Exploiting multiple exciton effects in organic solar cells

太阳代表着地球上最丰富的潜在可持续能源。利用有机导电聚合物将光能转化为电能的太阳能电池？有机光伏（OPV）装置——为可再生电力生产提供了一条潜在的低成本途径。然而，为了达到与其他太阳能光伏技术平价，有机太阳能电池必须提高其功率转换效率。该项目将光活性材料纳入有机聚合物太阳能电池装置中，以促进称为三重态融合的量子力学过程。人们认为，三重态聚变可以提高输出功率的电压，从而提高太阳能转换效率。从科学的角度来看，这个概念是创新的，因为它将专注于增加电压电位，而不是光电流，从而通过提高热力学效率来提高太阳能电池的效率。与该项目相关的教育活动包括参与普林斯顿大学材料学院，向来自新泽西州特伦顿的代表性不足的高中生教授动手材料科学，以及通过在当地图书馆举办的纳米材料科学日的多代外展活动，鼓励学生、教师和家长与太阳能科学家会面，并参与动手演示，制作和测试简单的太阳能电池设备。该研究的总体目标是开发一种新的基于有机聚合物的激子太阳能光伏器件架构，该架构有可能通过三重态聚变过程超越经典的热力学功率转换效率限制。在三重态核聚变中，由两个低能子带隙光子形成的两个激子需要产生一个高能光子。将敏化磷光材料集成到有机光伏（OPV）器件架构中提供了实现三重态融合的手段。在磷敏化OPV装置中，吸收和单线态形成主要发生在荧光宿主供体上。这个最初的吸收事件之后，能量转移到一个磷光客体存在于供体层在低浓度。然后，磷光客体上的激子转移到宿主的三重态水平，允许荧光材料中的长寿命三重态的人口。这种三重态聚变过程将被用来产生比载流子产生的激子能量更高的自由载流子。本质上，这种装置的结构代表了中间带太阳能电池的分子类似物，但具有与单线态裂变太阳能电池相似的极限效率。三重态聚变装置与传统的中间带太阳能电池的主要区别在于，在三重态聚变装置中，中间带是通过两个低能的三重态激子聚变成一个高能的单重态激子来满足的。此外，单线态裂变装置似乎增加了光电流，而基于三重态聚变的装置似乎增加了光电压，从而从根本上提高了热力学效率。为了探索这一新现象，本研究有两个主要目标。第一个目标是证明磷敏化OPV器件实现三重态融合的功能，第二个目标是通过全面的电学和光学表征来了解涉及多个激子器件的效率限制机制。最终，三态融合OPV装置的演示为超越Shockley-Queisser限制提供了另一种选择，并激发了对高效有机太阳能电池的研究。

The sun represents the most abundant potential source of sustainable energy on earth. Solar cells that use organic conducting polymers to convert light to electricity ? organic photovoltaic (OPV) devices - offer a potentially low-cost route for renewable electricity production. However, in order to achieve parity with other solar photovoltaic technologies, organic solar cells must increase their power conversion efficiency. This project will incorporate light-active materials into the organic polymer solar cell device to promote a quantum mechanical process called triplet fusion. It is believed that triplet fusion can increase the voltage of the power output, leading to higher solar energy conversion efficiency. This concept is innovative from a scientific point of view because it will focus on increasing the voltage potential, not the photocurrent, leading to a pathway to increase solar cell efficiency through an increase in thermodynamic efficiency. The educational activities associated with this project include participation in the Princeton University Materials Academy that teaches hands-on materials science to under-represented high school students from the Trenton, New Jersey, and multi-generational outreach through the Nano Materials Science Day at a local library that encourages students, teachers, and parents to meet with solar scientists and participate in hands-on demos to make and test simple solar cell devices. The overall goal of the research is to develop a new, organic polymer-based excitonic solar photovoltaic device architecture that has the potential exceed classical thermodynamic power conversion efficiency limits through the process of triplet fusion. In triplet fusion, two excitons formed from two low-energy sub band gap photons are required to produce one higher-energy photon. The integration of sensitized phosphorescent materials into the organic photovoltaic (OPV) device architecture offers the means to achieve triplet fusion. In a phosphor-sensitized OPV device, absorption and singlet formation occurs primarily on a fluorescent host donor. This initial absorption event is followed by energy transfer to a phosphorescent guest present in the donor layer at low concentration. Excitons on the phosphorescent guest then transfer to the triplet level of the host, permitting the population of the long-lived triplet state in the fluorescent material. This process of triplet fusion will be used to create free charge carriers with higher energy than the exciton from which the carriers originated. In essence, this device configuration represents the molecular analogue to an intermediate band solar cell, but possesses similar limiting efficiencies as singlet fission-based solar cells. The main distinction between the triplet fusion device and a conventional intermediate band solar cell is that, in the triplet fusion device, the intermediate band is satisfied via the fusion of two low energy triplet excitons into a higher energy singlet exciton. Furthermore, whereas the singlet fission device looks to increase photocurrent, the one based upon triplet fusion looks to increase the photovoltage, leading to fundamental enhancement of the thermodynamic efficiency. To explore this new phenomenon, the research has two major objectives. The first objective is to demonstrate the functionality of the phosphor-sensitized OPV device for achieving triplet fusion, and the second objective is to understand efficiency-limiting mechanisms involved with multiple exciton devices through comprehensive electrical and optical characterization. Ultimately, the demonstration of a triplet fusion OPV device adds another option to exceed Shockley-Queisser limits and inspire work toward high efficiency organic solar cells.